Such an injection molding machine with an injection unit is known, for example, from DE 101 14 006 C2. This injection unit includes as central “backbone” a so-called injection unit housing which normally has a solid, box or frame-like base structure. Attached to this injection unit housing is the plasticizing cylinder in which a screw is supported for rotation to prepare the plastic injection compound and for displacement to execute its injection stroke in axial direction. The screw is coupled to drive motors to generate the rotation and axial movements. These drive motors are supported on the injection unit housing via a motor flange.
In order to monitor the process during plastics injection molding, it is imperative to have knowledge about the injection force generated by the screw during its injection stroke. Injection force measuring devices are basically known in the state of the art.
The injection molding machine according to the afore-mentioned DE 101 14 006 C2 includes for this purpose strain sensors in the form of strain gauges or piezoelectric elements which are arranged in an axial circumferential U-shaped depression to allow determination of a deformation caused by an expansion of the motor flange as a consequence of a reaction force resulting from the injection force of the screw. This reaction force is transmitted via the screw onto the motor shaft which drives the screw and is supported axially in the motor casing. The reaction force is transmitted via this support onto the motor casing and thus onto the flange.
The conventional injection force measuring device has various shortcomings. The motor flange on which the strain sensors are attached is a component within the injection unit and exposed to dynamic interferences, such as torsional and vertical vibrations, in particular during the plasticizing and injection operations. These are directly transferred onto the motor flange and thus falsify the measured signal of the strain sensors.
In addition, the attachment zone of the strain sensors undergoes great temperature fluctuations as a result of its immediate proximity to significant heat sources, such as the drive motors themselves and the motor bearings.
Finally, the conventional configuration of the injection force measuring device does not allow the use of commercially available, industrial standard tensile force sensors. In contrast thereto, the strain gauges to be used are more difficult to use and increase the risk of error, especially when incorrectly attached.